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Uncoupling Protein 1 Controls Reactive Oxygen Species in Brown Adipose Tissue Martin Jastrocha,B,C,1

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Uncoupling Protein 1 Controls Reactive Oxygen Species in Brown Adipose Tissue Martin Jastrocha,B,C,1 COMMENTARY COMMENTARY Uncoupling protein 1 controls reactive oxygen species in brown adipose tissue Martin Jastrocha,b,c,1 Brown adipose tissue (BAT) is an organ specialized to demonstrating that UCP1 ablation is about more than fuel nonshivering thermogenesis for the defense of thermogenesis. high body temperature of many eutherian mammals in Mitochondrial ROS production has a major func- the cold. Cold-induced sympathetic stimulation of tional impact in all cells that has led to the emergence brown adipocytes activates lipolysis, glucose uptake, of new theories and biological disciplines such as the and mitochondrial biogenesis, with the mitochondrial “free radical theory of aging” addressing the damage biogenesis providing a powerful cellular engine for by ROS in aging (5) and the idea of ROS being an heat generation. At the mitochondrial inner mem- important signaling molecule. The sites of ROS pro- brane, the energy of nutrients such as glucose and duction have been intensively explored in recent lipids is converted into a proton gradient, but instead years, demonstrating that there are 11 molecular sites of storing the potential energy in the generation of that are capable of producing ROS, at least in isolated ATP, uncoupling protein 1 (UCP1) catalyzes an in- mitochondria (6). The magnitude of ROS depends on ducible proton leak to release the energy of the the concentration of electron transport chain (ETC) proton gradient directly as heat. The central role of complexes and their redox state, with the latter being UCP1 for nonshivering thermogenesis in rodents has affected by the concentration of electrons that are been confirmed in knockout mice (1, 2), and the ma- donated by substrates to the ETC and by their ability jority of research efforts on BAT and UCP1 centers on to be passed on from complex to complex so as to the physiological consequences of thermogenesis, in- reduce oxygen finally to water. How effectively these cluding the regulation of body weight for treatment of electrons can be passed along the ETC also depends obesity and other metabolic diseases. Although it ap- on the proton gradient generated by the same ETC. pears that UCP1’s function to release the proton mo- The higher the proton gradient, the more this gradient tive force as the final energy conversion step may only would stall the ETC, with the electrons possibly “over- affect thermogenesis, reduction of the proton motive reducing” their carriers and prematurely escaping the force will inevitably impact other mitochondrial and ETC complexes toward oxygen, generating either su- cellular processes. Reducing proton motive force by peroxide or hydrogen peroxide (3). Thus, interspersed mitochondrial proton leak would alter the redox state literature claiming that ROS production is positively of the respiratory chain and potentially reduce the associated with the rate of electron flux and oxygen production of reactive oxygen species (ROS) (3). The consumption cannot be substantiated with the current absence of UCP1 in BAT could be problematic, given understanding of bioenergetics. the high concentration of respiratory chain complexes More than 20 y ago, it was already suggested that in cold-acclimated brown fat mitochondria and the reduction of the proton motive force by uncoupling minor concentrations of ATP synthase, with the latter agents impacts mitochondrial production of ROS (7). not being able to release the enhanced, cold-induced Endogenous proton leak was proposed as the mech- proton motive force. Although the physiological con- anism to reduce ROS (8), and a similar impact on ROS sequences in UCP1 knockout mice have been almost by UCP1 has been verified in isolated brown fat mito- exclusively attributed to thermogenesis, there may be chondria by two laboratories (9, 10). However, the in- further consequences on mitochondrial ROS biology volvement of UCP1 in ROS biology has been disputed and downstream effects that have been widely ignored. based mainly on conflicting studies, claiming that ROS In PNAS, Kazak et al. (4) explore cold-induced molecu- directly activates UCPs, thus providing an elegant feed- lar differences of BAT lacking UCP1 and show that mi- back mechanism to prevent excessive mitochondrial ROS tochondrial calcium buffering is compromised through levels (reviewed in ref. 11). Other studies have reported ROSproductioninanUCP1-dependentmanner, that this feedback mechanism does not exist (12–14) or is aInstitute for Diabetes and Obesity, Helmholtz Diabetes Center, Helmholtz Zentrum München, D-85764 Neuherberg, Germany; bGerman Center for Diabetes Research, D-85764 Neuherberg, Germany; and cDepartment of Animal Physiology, Faculty of Biology, Philipps University of Marburg, D-35032 Marburg, Germany Author contributions: M.J. wrote the paper. The author declares no conflict of interest. See companion article on page 7981. 1Email: [email protected]. 7744–7746 | PNAS | July 25, 2017 | vol. 114 | no. 30 www.pnas.org/cgi/doi/10.1073/pnas.1709064114 Downloaded by guest on September 23, 2021 decreased expression of genes involved in oxidative phosphoryla- tion, thus providing the molecular basis for previous functional observations demonstrating reduced maximal respiration rates in isolated UCP1 knockout mitochondria (10). Increased mitochon- drial ROS levels might cause the reduction of the ETC, because mainly mitochondrial, but not nuclear-encoded, ETC subunit ex- pression was disturbed. Importantly, Kazak et al. (4) point out that not all differences in mitochondrial respiration between wild-type and UCP1 knockout mice can be solely attributed to differences in mitochondrial proton leak. That notion is important because UCP1 knockout mice are now widely distributed to investigate BAT mechanisms for the treatment of metabolic diseases. Indeed, mitochondrial uncoupled respiration is only an indirect measure of the mitochondrial proton leak, because proton leak respiration is also a function of maximal substrate oxidation capacity. Only when maximal respiration is constant does the measurement of proton leak respiration give a semiquantitative readout for proton leak (20, 21). Thus, precise measurements of proton conductance would require the simultaneous assessment of proton motive force, which is still the gold standard to quantify the proton leak rates (22). Kazak et al. (4) observe enriched expression of innate immunity genes in UCP1-ablated BAT, supporting a relationship between the immune system and adaptive thermogenesis. So far, it remains un- clear whether these gene expression changes are only signs of dysfunctional cells or whether they would support nonshivering thermogenesis (23) or not (24). Kazak et al. (4) establish causality between mitochondrial calcium loading and ROS. UCP1-lacking BAT mitochondria appear more sensitive to extramitochondrial cal- Fig. 1. Regulation of mitochondrial ROS production in BAT. Increases cium levels that are not explained by differences in the permeability in ROS levels upon adrenergic stimulation in brown adipocytes have transition pore however (25). Strikingly, eliminating ROS production been reported by Chouchani et al. (18). In the wild-type condition, increased ROS levels are mitigated by higher UCP1 activity (9, 10). from the IQ site of respiratory complex I or using the antioxidant Although causality is not fully established, reducing ROS generation may MitoQ restores calcium handling of BAT mitochondria. Fig. 1 depicts enable an increase in ETC complexes, which is supported by UCP1- that ROS production from the IQ site is sensitive to mitochondrial dependent regulation of mitochondrial-encoded ETC subunits (4). uncoupling and the ROS production by reverse-electron transfer Whether the responses of the innate immune system can be attributed to (RET) from this site is blocked by using rotenone in the current study. ROS remains to be elucidated. In this study, Kazak et al. (4) establish causality between reduced mitochondrial calcium buffering and Clearly, the lack of UCP1 has more than just thermogenic increased ROS in UCP1-ablated BAT mitochondria. Mitochondrial ROS consequences. It would be an important goal to assess all non- production is substantially reduced by UCP1 activity at the brown fat thermogenic effects of UCP1 for both fundamental research and mitochondrial inner membrane [values are calculated using the method translational purposes. How the presence of UCP1 affects cellular of Oelkrug et al. (10)]. Kazak et al. (4) use the complex I inhibitor rotenone processes in human adipose tissue, or in the newly identified UCP1- to eliminate proton motive force dependent ROS at the IQ site and reduce overall ROS production by mitochondrially targeted antioxidant positive beige adipocyte type, still remains unknown. Given some MitoQ, which are both treatments fully rescuing compromised doubts about the thermogenic role of beige adipose tissue, could −/− mitochondrial calcium loading in UCP1 mitochondria.ETF,electron its recruitment of UCP1 by a variety of substances and stress factors transferring flavoprotein; G3P, glycerol-3-phosphate; Q, coenzyme Q. just be an antioxidant response? Kazak et al. (4) highlight that there is more than thermogenesis to mediated by other carriers, such as the adenine nucleotide translocase the elimination of UCP1 in mice that needs to be considered when (ANT)(15).Thesuperoxidedismutase2(SOD2)overexpressormouse, performing experiments using these mice. One could also consider exhibiting reduced superoxide levels, represented
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